How does fly.io calculate VM exec time?

Hey folks, I used fly.io to deploy my personal blog (which is a static site powered by Hugo). The free allowances says the entitlement is

2,340 hours per month of VM: shared-cpu.

The app has been running for close to 18 hours now, and from the usage screen, I see Firecracker VM exec time as only 6 hours. Is “idle” time not considered? Or is it that the VMs shutdown when not in use? If so, are there any “cold start” penalties?

Is “idle” time not considered?

Only folks from Fly.io can confirm, but this answer here by Greg has some pointers: Understanding FIRECRACKER LOAD AVERAGE.

Or is it that the VMs shutdown when not in use? If so, are there any “cold start” penalties?

No, there are not. That said, shared-cpu-1x is (?) ~1/15th CPU slice: shared-cpu-1x performance on CPU bound tasks.

Btw, “serverless” is a planned (future) feature with ~500ms cold starts, if Kurt is to be believed. Tease us with more "machine" info?


A AWS Community Hero deploying their personal blog on Fly.io? That’s a first :slight_smile:

Thanks for the links, I’ll give them a read. I see the VM exec time is updated, albeit slowly, would be nice to see it updated faster.

My only regret is that it took me so long to try it out ;D

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